Scientists have some ideas about why we haven’t found aliens yet. They think there are two possibilities: either there is a lot of life in the universe, or almost no life at all. A study published in the International Journal of Astrobiology gives some reasons for this mystery.
Scientists searching for extraterrestrials believe that many more advanced civilizations should exist besides our own. They used math and logic, based on the work of John Haldane, to explain why we haven’t found alien civilizations yet.
Imagine a group of planets like Earth. You would expect life to appear on all of them, or on none of them. There is no obvious reason why only half of these planets would have life.
This idea can be represented in a U-shaped graph: on one side there is a high probability that there are no planets, and on the other side there is a high probability that there are a lot of life. In the middle, it is unlikely that only a few planets have life.
The Drake Equation: What is it?
Scientists used Haldane’s logic to change the famous Drake equation. Astronomer Frank Drake made this comparison to guess how many alien civilizations might be in the Milky Way.
The Drake equation looks like this: N = R* x fp x ne x fl x fi x fc x L
Astronomers know a lot about star formation and how many stars have planets, but they are less certain about how many planets could support life. The rest of the equation is still a mystery, so scientists are left to guess.
The study’s authors say the first six parts of the Drake Equation are about how alien civilizations might arise. The last part, L, is about when these civilizations might disappear.
A new formula for counting alien civilizations
By breaking down the parts of the Drake equation, scientists have found a simpler way to look at it: the average number of alien civilizations in the Milky Way is equal to the rate at which they are born multiplied by the rate at which they die. This means scientists don’t have to worry too much about the unknown parts of the Drake equation.
They also added a new idea called the population fraction, F. A high F, close to 1, would mean that almost every planet has life. A low F, close to 0, would mean that there are very few, if any, planets with life.
The problem is that, based on what scientists have seen, F is probably not close to 1. If that were the case, we would already have found aliens building things or sending radio signals into space.
This means that if we are not alone in the universe, F could be closer to 0.5, which puts us in the unlikely part of the U-shaped graph. This suggests that intelligent life is rare in the universe.
Scientists believe that if alien civilizations had ever existed in the Milky Way, they could have spread across the Milky Way within 10 to 100 million years. Even if they disappeared we should still find evidence of their existence. Since we haven’t found anything, it seems that alien civilizations are rare and don’t last long.
If aliens exist, why haven’t we found them?
The authors also suggest that if we ignore the lack of evidence, the Drake equation shows that the universe can be full of life or nearly empty.
If the universe were full of life, F would be close to 1, and this could still be possible under certain conditions.
Maybe alien civilizations stay in their own part of space, and our solar system isn’t a place they visit. Maybe traveling between stars is too difficult, or maybe they just don’t have to do it.
It is also possible that while there is a lot of complex life in the universe, there are very few technologically advanced civilizations, or they take a long time to develop.
There’s even a chance that the birth and death rates of alien civilizations aren’t in balance yet, so we might still have time to find them. But given how old the universe is, this seems unlikely.
